Nonvolatile logic and security circuits

ABSTRACT

In some examples, a nonvolatile storage element may be configured to store a state or value during a low power or powered down period of a circuit. For example, the nonvolatile storage element may include a bridge of resistive elements that have a resistive state that may be configured by applying voltages to multiple drive paths. A sense amplifier may be connected to the bridge in order to resolve a voltage differential associated with the bridge to either power or ground and, thereby determine the state associated with on the nonvolatile storage element.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional and claims priority to U.S. patentapplication Ser. No. 14/495,678, filed on Sep. 24, 2014, which claimspriority to U.S. Provisional Patent Application No. 61/951,002 filedMar. 11, 2014.

BACKGROUND

Circuits typically experience a static current draw when power isapplied due to leakage. However, powering a circuit down may beproblematic as the circuitry is unable to retain the current state uponpower up without lengthily power up or initialization sequences becausethe latch elements are typically volatile storage elements and disablingpower would cause the loss of the state stored on the latch.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is described with reference to the accompanyingfigures. In the figures, the left-most digit(s) of a reference numberidentifies the figure in which the reference number first appears. Theuse of the same reference numbers in different figures indicates similaror identical components or features.

FIG. 1 illustrates a partial logic diagram and partial circuit diagramshowing select components of circuit for maintaining a state through apower off period according to some implementations.

FIG. 2 illustrates a physical diagram showing select components of thebridge of FIG. 1 according to some implementations.

FIG. 3 illustrates a partial logic diagram and partial circuit diagramshowing select components of the sense amplifier of FIG. 1 according tosome implementations.

FIG. 4 illustrates a partial logic diagram and partial circuit diagramshowing select components of circuit for maintaining a state through apower off period according to some implementations.

FIG. 5 illustrates a physical diagram showing select components of thebridge of FIG. 2 according to some implementations.

FIG. 6 illustrates a partial logic diagram and partial circuit diagramshowing select components of the sense amplifier of FIG. 4 according tosome implementations.

FIG. 7 illustrates a partial logic diagram and partial circuit diagramshowing select components of circuit for use a physical unclonablefunction (PUF) according to some implementations.

FIG. 8 illustrates a block diagram of a device utilizing a PUF as partof an unlock key according to some implementations.

FIG. 9 illustrates an example flow diagram showing an illustrativeprocess for maintaining a state associated with a nonvolatile storageelement through a powered down period according to some implementations.

FIG. 10 illustrates an example flow diagram showing an illustrativeprocess for utilizing a nonvolatile storage element as a PUF accordingto some implementations.

DETAILED DESCRIPTION

This disclosure includes techniques and implementations to implement anonvolatile storage element. In some examples, the magnetic randomaccess memories (MRAMs) or spin torque MRAM (ST-MRAM) elements may beutilized to generate nonvolatile storage elements, such as nonvolatileflip flop, logic gate, or other circuit components for maintain a stateassociated with a circuit while a device or portion of a device ispowered off. For instance, in some implementations, the nonvolatilestorage elements described herein may utilize magnetic orientationopposed to a charge state to maintain a value.

In one implementation, a nonvolatile storage element may include aplurality of restive elements or magnetic tunnel junctions (MTJs)arranged as a bridge having a pair of midpoints with pairs of MTJspositioned both above and below each of the midpoints. In someimplementations, the number of pairs of MTJs above each midpoint andbelow each midpoint are equal. The nonvolatile flip flop also includes aread bias/write driver circuit connected to the bridge and configured togenerate drive signals to cause the pairs of MTJs to be driven high orlow, such that a value or state may be stored on the bridge.

For instance, in one particular implementation, the bridge includes twopairs (or four) MTJs above the midpoints and two pairs (or four) MTJsbelow the midpoints. In this implementation, the read bias/write drivercircuit connected to the bridge and configured to generate four drivesignals (e.g., two above the midpoints and two below the midpoints) tocause the pairs of MTJs to be driven high and/or low, such that aparticular voltage differential associated with the midpoints of thebridge and thereby a state associated with the particular voltagedifferential may be stored.

In some implementations, described herein, the nonvolatile storageelement is connected to a sense amplifier that is configured to receivean output value associated with the MTJs and to determine the state ofthe bridge based on a differential associated with the output values andthereby a value stored on the bridge. For instance, the bridge mayinclude a first midpoint voltage associated with a first column of thebridge and a second midpoint voltage associated with the second columnof the bridge and the sense amplifier may determine the value of thebridge based on a comparison between the first midpoint voltage and thesecond midpoint voltage.

In another particular implementation, the bridge includes four pairs (oreight) MTJs above the midpoints and four pairs (or eight) MTJs below themidpoints. In this implementation, the read bias/write driver circuitconnected to the bridge and configured to generate eight drive signals(e.g., four above the midpoints and four below the midpoints) to causethe pairs of MTJs to be driven high and/or low, such that a firstvoltage differential may be associated with the lower half of the bridgeand a second voltage differential may be associated with the upper halfof the bridge.

In this implementation, a sense amplifier may be able to determine thestate associated with the bridge and the value stored on the nonvolatilestorage element based on the first and second voltage differentials. Forinstance, the state may be determined based in part on voltagesassociated with the upper half of the first column, the lower half ofthe first column, the upper half of the second column, and the lowerhalf of the second column.

In some implementations, due to the fact that the stored magneticorientations associated with the nonvolatile storage elements aredifficult to detect, the nonvolatile storage elements may be utilized togenerate one or more security codes or keys for a device, such as adevice specific fingerprint. For instance, in some implementations,multiple nonvolatile storage elements may be used to create individualdigits of a physical unclonable function (PUF) that may be used as asignature or fingerprint unique to each device. Since each digit of thePUF signature is based on a nature variation in the resistance of eachstate of the magnetic storage elements (e.g., the MTJs) associated withthe nonvolatile storage elements, the signatures are not easilyidentified or duplicated. In some specific implementations, an errorcorrection code (ECC) circuit may be used in conjunction with the PUFcircuit to improve the consistency of the output response and, thereby,the overall reliability of the PUF circuit.

FIG. 1 illustrates a block diagram showing select components of circuit100 for maintaining a state through a power off period according to someimplementations. In the illustrated example, the circuit 100 includes avolatile storage element 102 and a nonvolatile storage element 104. Ingeneral, the volatile storage element 102 may be employed to store astate or data associated with the circuit 100, while the circuit 100 ispowered on. However, in some implementations, the volatile storageelement 102 may be unable to maintain the data or the state in responseto the circuit 100 being powered down. Accordingly, in the event thatpower to the circuit 100 is reduced, such as when the circuit 100 istransitioned from an active state to a low power state or an off state,the nonvolatile storage element 104 is capable of reading a value or astate associated with the volatile storage element 102 and maintainingthe value and/or the state until the power to the circuit 100 isreestablished.

The nonvolatile storage element 104 may include a bridge, generallyindicate by 106, having a bridge top 108 and a bridge bottom 110. Thebridge 106 may be formed from multiple resistive elements, such astunnel junctions or MTJs, arranged in two columns. For instance, in theillustrated example, the bridge 106 includes a first column of MTJs112-118 and a second column of MTJs 120-126. In the illustrated example,the MTJs 112-126 are arranged such that four of the MTJs 112, 114, 120and 122 are arranged above a pair of midpoints associated with thebridge 106 and four MTJs 116, 118, 124, and 126 are arranged below themidpoints. By arranging the MTJs 112-126 in this manner, a value (e.g.,one or zero) may be determined from the voltage differential associatedwith the midpoints of the bridge.

Each of the MTJs 112-126 may be driven to a high state or a low stateand the state of each of the respective MTJs 112-126 may be based on arelative magnetic spin associated with the magnetic layers of the MTJs112-126. In some implementations, the magnetic spin associated with theMTJs 112-126 may be set by applying voltages or drive signals 128-134 tothe MTJs 112-126 of the bridge 106. In some cases, the state of the MTJs112-126 may be set to produce one or more particular arrangements on thebridge 106. Each of the particular arrangements result in a particularvoltage differential associated with the midpoints of the bridge 106.For instance, in one particular arrangement, a voltage associated withthe midpoint of the first column of the bridge 106 may be greater than avoltage associated with the midpoint of the second column of the bridge106. In another particular arrangement, the voltage associated with themidpoint of the first column of the bridge 106 may be less than avoltage associated with the midpoint of the second column of the bridge106

In general, the volatile storage element 102 has a state or value (e.g.,zero or one) while the circuit 100 is powered on. In response to thecircuit 100 being powered down or placed in a low power state, forinstance, to conserve power when the circuit is not in use by theelectronic device, the a read bias/write driver 136 may receive a storesignal 148 and the state of the volatile storage element 102. Dependingon the state, the read bias/write driver 136 provides the drive signals128-130 to the bridge 106 to set the state of the MTJs 112-126.

For example, when the value to be stored by the nonvolatile storageelement 104 is a zero, the read bias/write driver 136 may drive signal128 high along drive path 152, drive signal 130 low along drive path154, drive signal 132 low along drive path 156, and drive signal 134high along drive path 158. In this example, the drive signals 128 and130 cause the MTJ 112 to a high state and MTJ 120 to a low state, thedrive signals 130 and 134 caused the MTJ 122 to a low state and MTJ 124to a high state, the drive signals 132 and 134 cause MTJ 118 to a lowstate and MTJ 126 to a high state, and the signals 128 and 132 causedthe MTJ 114 to a high state and MTJ 116 to a low state. Alternatively,when the value to be stored by the nonvolatile storage element 104 is aone, the read bias/write driver 136 may cause drive signal 128 low alongdrive path 152, drive signal 130 high along drive path 154, drive signal132 high along drive path 156, and drive signal 134 low along drive path158. In this example, the drive signals 128 and 130 cause the MTJ 112 toa low state and MTJ 120 to a high state, that drive signals 130 and 134cause the MTJ 122 to a high state and MTJ 124 to a low state, the drivesignals 132 and 134 cause MTJ 118 to a high state and MTJ 126 to a lowstate, and the drive signals 128 and 132 cause the MTJ 114 to a lowstate and MTJ 116 to a high state.

In this example, by setting the MTJs 112-126 in the arrangementsdescribed above, a value (e.g., high or low corresponding to zero orone), may be detected by reading a value associated with the midpoint,generally indicated by 140 and 142, of each column of the bridge 106.For instance, in the illustrated example, the bridge 106 has a first outpath P 160 associated with the midpoint 140 of the first column and asecond out path N 162 associated with the midpoint 142 of the secondcolumn. In general, the sense amplifier 138 may receive an out P signal144 associated with the first column and an out N signal 146 associatedwith the second column.

When power is restored to the volatile storage element 102, the senseamplifier 138 and the read bias/write driver 136 receive the recallsignal 150, and in response, the read bias/write driver 136 biases thebridge top 108 and the bridge bottom 110. Once, the bridge top 108 andthe bridge bottom 110 are biased, the sense amplifier 138 is able todetect a differential voltage associated with the out signals 144 and146. The sense amplifier 138 is then able to determine a stateassociated with the nonvolatile storage element 104 based on thedifferential voltage. For example, when reading the value stored on thebridge 106, the read bias/write driver 136 bias the bridge top 108 bygenerating a top signal 160 and the bridge bottom 110 by generating abottom signal 162. In response, a voltage may be detected at midpoints140 and 142 as part of out signals 144 and 146 by the sense amplifier138. The sense amplifier 138 may determine the value (e.g., either azero or one), as will be described in more detail below with respect toFIG. 3, based in part on which of out signal 144 or out signal 146 has agreater voltage.

For instance, the sense amplifier 138 includes a pair of cross coupleddevices (not shown) that are able to resolve the voltage differentialbetween the out signals 144 and 146 to either power or ground and,thereby produce a digital output that may be provided back to thevolatile storage element 102. Thus, in one particular example, if theMTJs have a resistance of two when in a high state and one when in a lowstate and the value to be stored is a zero, the MTJs 112, 114, 124 and126 have a resistance of two and the MTJs 116, 118, 120, and 122 have avalue of one. When the bridge top 108 and the bridge bottom 110 arebiased by the read bias/write driver 136 to VDD, the midpoint 140 has avalue of 0.33 times VSS and the midpoint 142 has a value of 0.66 timesVDD. The values on each midpoint 140 and 142 may be detected by thesense amplifier 138 and, since midpoint 142 has a higher voltage thanthe midpoint 140, the cross coupled device may cause the voltages toresolve towards ground. In response to identifying that the voltages arebeing resolved towards ground, the sense amplifier 138 is able toprovide the value of zero back to the volatile storage element 102.

Likewise, in another particular example, if the MTJs have a resistanceof two when in a high state and one when in a low state and the value tobe stored is a one, then the MTJs 112, 114, 124 and 126 have aresistance of one and the MTJs 116, 118, 120, and 122 have a value oftwo. When the bridge top 108 and the bridge bottom 110 are biased by theread bias/write driver 136 to VDD, the midpoint 140 has a value of 0.66times VDD and the midpoint 142 has a value of 0.33 times VDD. The valueson each midpoint 140 and 142 may be detected by the sense amplifier 138and, since midpoint 140 has a higher value than the midpoint 142, thecross coupled device may cause the voltages to resolve towards VDD. Inresponse to identifying that the voltages are being resolved towardsVDD, the sense amplifier 138 is able to provide the value of one back tothe volatile storage element 102.

In the present example, when the sense amplifier 138 resolves thevoltage differential to ground a value of zero is detected and when thesense amplifier 138 resolves the voltage differential to power a valueof one is detected. However, it should be understood that in otherexamples, the sense amplifier 138 may assign the value of zero to avoltage differential that resolves towards power and a value of one to avoltage differential that resolves towards ground.

In the present example, the bridge 106 includes eight MTJs 112-126.However, in some implementations, the number of resistive elements orMTJs associated with the bridge 106 are related to the number of MTJsrequired to distribute the voltages associated with the drive signals128-132 between the MTJs without causing one or more of the valuesstored on the MTJs to be flipped or switched. For instance, a bridgeincluding sixteen MTJs is described below with respect to FIG. 4.

FIG. 2 illustrates a physical diagram showing select components of thebridge 106 of FIG. 1 according to some implementations. In theillustrated example, the MTJs 112-126 are shown along a metal layer oras part of a physical layout including the interconnections via thebridge top 108, the bridge bottom 110, drive 152-158, out path P 160,and out path N 162. For instance, the MTJ 118 and the MTJ 116 areconnected via the drive path 152, the MTJ 116 and the MTJ 114 areconnected via the out path P 160, the MTJ 114 and the MTJ 112 areconnected via the drive path 154, the MTJ 112 and the MTJ 120 areconnected via the bridge top 108, the MTJ 120 and the MTJ 122 areconnected by the drive path 156, the MTJ 122 and the MTJ 124 areconnected via the out path N 162, the MTJ 124 and the MTJ 126 areconnected via the drive path 158, and the MTJ 126 and the MTJ 118 areconnected via the bridge bottom 110.

In general, when a value of zero is to be stored by the bridge 106, awrite driver, such as the read bias/write driver 136 of FIG. 1, causesthe drive path 152 to be driven low and drive path 154 to be driven highresulting in the MTJ 116 being placed in a low state and MTJ 114 in thehigh state, the drive path 156 to be driven low with drive path 154driven high resulting in the MTJ 120 being placed in a low state and MTJ112 being placed in the high state, the drive path 158 to be driven highwith drive path 156 driven low, resulting in the MTJ 124 being placed ina high state, and MTJ 122 being placed in the low state, and drive path158 driven high with drive path 152 driven low results in the MTJ 126being placed in the high state and MTJ 118 being placed in the lowstate. In other words, when the value of zero is to be stored by thebridge 106, the drive path 152 and the drive path 156 are driven low,and the drive path 154 and the drive path 158 are driven high therebyapplying a down current along the MTJs 116, 118, 120, and 122 and an upcurrent along the MTJs 112, 114, 124, and 126.

Likewise, when a value of one is to be stored by the bridge 106 thewrite driver causes the drive path 152 to be driven high and drive path154 to be driven low resulting the MTJ 116 being placed in a high stateand MTJ 114 being placed in the low state, the drive path 156 to bedriven high with drive path 154 driven low resulting the MTJ 120 beingplaced in a high state and MTJ 112 being placed in the low state, thedrive path 158 to be driven low with the drive path 156 driven highresulting the MTJ 124 being placed in a low state and MTJ 122 beingplaced in the high state, and drive path 158 driven low with drive path152 driven high results in the MTJ 126 being placed in the low state andMTJ 118 being placed in the high state. In this way, the write driver isable to configure the MTJs 112-126 associated with the bridge 106 tomaintain a value of either zero or one when power associated with thecircuit is off. In other words, when the value of one is to be stored bythe bridge 106, the drive path 152 and the drive path 156 are drivenhigh, and the drive path 154 and the drive path 158 are driven lowthereby applying an up current along the MTJs 116, 118, 120, and 122 anda down current along the MTJs 112, 114, 124, and 126.

In some implementations, by setting configuring the resistive state ofthe MTJs 112-126 by applying voltages to the drive paths 152-158 in themanner described above, the bridge 106 may be configured to store avalue (e.g., one or zero) detectable by a sense amplifier, such as thesense amplifier describe below with respect to FIG. 3, when the bridgetop 108 and the bridge bottom 110 are biased by a read bias component,such as read bias/write driver 136 of FIG. 1.

For instance, in the illustrated example, the read bias component maydrive the bridge top 108 high and the bridge bottom 110 low associatinga first voltage with the out path P 160 and a second voltage with theout path N 162 based on the resistive states of the MTJs 112-126, asconfigured in response to the write driver applying various voltages tothe drive paths 152-158. In this instance, a first voltage may bedetected on the out path P 160 and a second voltage may be detected onthe out path N 162 by one or more sense amplifiers. The sense amplifiermay determine the value stored on the bridge 106 based at least in parton the differential between the first voltage and the second voltage.For example, the sense amplifier may include one or more cross coupleddevices configured to cause the voltage differential between the firstvoltage and the second voltage to resolve to either power or ground,and, thereby the sense amplifier may output a digital value, such as azero or one corresponding to either power or ground.

FIG. 3 illustrates a partial logic diagram and partial circuit diagramshowing select components of the sense amplifier 138 of FIG. 1 accordingto some implementations. In general, the sense amplifier 138, inresponse to receiving an enable signal 302 (such as the recall signal150 of FIG. 1), is configured to determine or detect a value stored onthe bridge associated with the nonvolatile storage element of FIGS. 1and 2 when the bridge top and bridge bottom are biased by a read biascomponent based at least in part on the voltage differential between thevoltage associated with the out path P 144 and the out path N 146 ofFIG. 1. For instance, in the illustrated example, the sense amplifier138 receives an out P signal 144 associated with the voltage on the outpath P 160 at an impedance path 304 and an out N signal 146 associatedwith the voltage on the out path N 162 at an impedance path 306 andbased on the differential between the out P signal 144 and the out Nsignal 146, as will be described in more detail below, determine adigital value (e.g., zero or one) associated with nonvolatile storageelement 102.

In the illustrated example, the sense amplifier 138 includes two crosscoupled devices 308 and 310 coupled to the impedance paths 304 and 306.The cross coupled devices 308 and 310 are arranged such that based onthe voltage differential between out P signal 144 and out N signal 146,impedance paths 304 and 306 resolve to either ground or power (e.g.,zero or one). In this manner, the sense amplifier 138 may output a dataout signal 312 as a digital signal of either zero or one to the volatilestorage element 102 upon power up of the electronic device.

In one example, assume that out P signal 144 is higher than the out Nsignal 146. In this example, the voltage received at a transistorcontrolling the impedance path 304 is larger than the voltage receivedat a transistor controlling the impedance path 306 causing a voltageassociated with the impedance path 304 to be pulled down further than avoltage at the impedance path 306. Thus, the voltage associated with theimpedance path 304 is lower than the voltage associated with theimpedance path 306. When this happens, a feedback loop associated withthe cross coupled devices 308 and 310 cause the voltage associated withimpedance path 304 to tend towards ground and the voltage associatedwith the impedance path 306 to tend towards power, which the senseamplifier 138 detects as a high signal or a one that may be output aspart of data out signal 312.

In an alternative example, assume that out P signal 144 is lower thanthe out N signal 146. In this example, the voltage received at thetransistor controlling the impedance path 304 is lower than the voltagereceived at the transistor controlling the impedance path 306 causing avoltage associated with the impedance path 306 to be pulled down furtherthan a voltage at the impedance path 304. Thus, the voltage associatedwith the impedance path 304 is higher than the voltage associated withthe impedance path 306. When this happens, the feedback loop associatedwith the cross coupled devices 308 and 310 cause the voltage associatedwith impedance path 304 to tend towards power and the voltage associatedwith the impedance path 306 to tend towards ground, which the senseamplifier 138 detects as a low signal or a zero that may be output aspart of data out signal 312. Thus, in this manner the sense amplifier138 is capable of translating the high and low signals stored on theMTJs of the bridge associated with the nonvolatile storage element intoa digital signal that may be provided to the volatile storage element102 when power is restored to the circuit.

The sense amplifier 138 in the illustrated example provides one possibleimplementation of a sense amplifier configured to sense a value (eitherzero or one) stored on the nonvolatile storage elements of FIGS. 1 and2. However, it should be understood that other implementations of asense amplifier may be utilized to sense or detect the value stored onthe bridge, for instance, the sense amplifier described below withrespect to FIG. 6.

FIG. 4 illustrates a partial logic diagram and partial circuit diagramshowing select components of circuit 400 for maintaining a state througha power off period according to some implementations. As describedabove, in the illustrated example, the circuit 400 includes a volatilestorage element 402 and a nonvolatile storage element 404. In general,the volatile storage element 402 may be employed to store a state orvalue associated with the circuit 400, while the circuit 400 is poweredon. However, in some implementations, the volatile storage element 402may be unable to maintain the data or the state in response to thecircuit 400 being powered down. Accordingly, in the event that power tothe circuit 400 is reduced, such as when the circuit 400 is transitionedfrom an active state to a low power state or an off state, thenonvolatile storage element 404 is capable of reading the data or thestate from the volatile storage element 402 and maintaining the dataand/or the state until the power to the circuit 400 is reestablished.

The nonvolatile storage element 404 may include a bridge, generallyindicate by 406, having a bridge top 408 and a bridge bottom 410. Thebridge 406 may be formed from multiple resistive elements, such astunnel junctions or MTJs 412-442, arranged in two columns. For instance,in the illustrated example, the bridge 406 includes a first column ofthe MTJs 412-426 and a second column of the MTJs 428-442. In theillustrated example, the MTJs 412-442 are arranged such that eight ofthe MTJs 412-418 and the MTJs 428-434 are arranged above midpoints,generally indicated by 440 and 442, associated with the bridge 406 andeight MTJs 420-426 and 436-442 are arranged below the midpoints 440 and442.

The illustrated example, may include sixteen MTJs 412-442 instead of theeight MTJs 112-126 associated with the bridge 106 of FIG. 1, as in someelectronic devices, the bias voltages and when distributed across theeights MTJs may be sufficient to disturb the resistive state associatedwith one or more of the eight MTJs. Therefore, in some implementations,additional MTJs may be associated with the bridge, as illustrated in thecurrent example.

In the illustrated examples, the bridge 406 is arranged, such that astate or value may be determined from a first differential voltageassociated with the top half of the bridge 406 and a second differentialvoltage associated with the bottom half of the bridge 406. For instance,the nonvolatile storage element 404 includes an out path P1 446 and anout path N1 448 associated with the top half of the bridge 406, as wellas an out path P2 450 and an out path N2 452 associated with the bottomhalf of the bridge 406. In some cases, a sense amplifier 454 by beconfigured to detect a voltage associated with the out paths 446-452 andbased on the differential values resolve the voltages to ground or power(e.g., a value of zero or one), as will be described in more detail withrespect to FIG. 6 below.

In some examples, each of the MTJs 412-442 may be driven to a high stateor a low state. In some implementations, a magnetic spin associated withthe MTJs 412-442 may be set by driving voltages (either high or low)over drive paths 454-470. In some cases, the state of the MTJs 412-442may be set or arranged to produce one or more particular arrangements onthe bridge 406. Each of the particular arrangements results in aparticular voltage differential that may be detected by the senseamplifier 454.

In one instance, a read bias/write driver 472 may receive a store signal476 that may cause the read bias/write driver 472 to drive a highvoltage over the drive paths 456, 462, 464, and 470 and a low voltageover the drive paths 458, 460, 466, and 468 for instance in order tostore a value of zero. In this example, the high voltages associatedwith the drive paths 456, 462, 464, and 470 place and the low voltagesassociated with the drive paths 458, 460, 466, and 468 place the MTJs412, 414, 420, 422, 432, 434, 440, and 442 in a high state and the MTJs416, 418, 424, 426, 428, 430, 436, and 438 in a low state.

Alternatively the read bias/write driver 472 may receive a store signal476, that may cause the read bias/write driver 472 to drive a lowvoltage over drive paths 456, 462, 464, and 470 and a high voltage overthe drive paths 458, 460, 466, and 468, for instance in order to store avalue of one. In this example, the low voltages associated with thedrive paths 456, 462, 464, and 470 and the high voltages associated withthe drive paths 458, 460, 466, and 468 place the MTJs 412, 414, 420,422, 432, 434, 440, and 442 in a low state and the MTJs 416, 418, 424,426, 428, 430, 436, and 438 in a high state.

In this example, by setting the MTJs 412-442 in the arrangementsdescribed above, a value may be detected by sensing the voltagesassociated with the out paths 446-452 when a bias is applied to thebridge top 408 and the bridge bottom 410. For instance, when power isrestored to the volatile storage element 402, the sense amplifier 454and the read bias/write driver 472 may receive the recall signal 474,and in response, the read bias/write driver 472 biases the bridge top408 and the bridge bottom 410. Once, the bridge top 408 and the bridgebottom 410 are biased, the sense amplifier 454 is able to detect avoltage associated with the out signals 446-452 and, based on thevoltages, determine the value stored on the nonvolatile storage element404.

For instance, the sense amplifier 454 may include a pair of crosscoupled devices (not shown) that are able to resolve the voltagedifferentials associated with the out paths 446-452 to produce a digitaloutput that may be provided back to the volatile storage element 402.For example, in one particular instance, if the MTJs have a resistanceof two when in a high state and one when in a low state and the value tobe stored is a zero, then the MTJs 412, 414, 420, 422, 432, 434, 440,and 442 have a resistance of two and the MTJs 416, 418, 424, 426, 428,430, 436, and 438 have a value of one. When the bridge top 408 and thebridge bottom 410 are biased by the read bias/write driver 472, the outpath P1 446 has a voltage of 0.66, the out path N1 448 has a voltage of0.83, the out path P2 450 has a voltage of 0.16, and the out path N2 452has a voltage of 0.33. The voltages associated with each the out paths446-452 may be detected by the sense amplifier 454 and the cross coupleddevice may cause the voltages to resolve towards ground. In response toidentifying that the voltages are being resolved towards ground, thesense amplifier 454 is able to provide the value of zero back to thevolatile storage element 402.

Likewise, in another particular instance, if the MTJs have a resistanceof two when in a high state and one when in a low state and the value tobe stored is a one, then MTJs 412, 414, 420, 422, 432, 434, 440, and 442have a resistance of one and MTJs 416, 418, 424, 426, 428, 430, 436, and438 have a value of two. When the bridge top 408 and the bridge bottom410 are biased by the read bias/write driver 472, the out path P1 446has a voltage of 0.83, the out path N1 448 has a voltage of 0.66, theout path P2 450 has a voltage of 0.33, and the out path N2 452 has avoltage of 0.16. The voltages associated with each the out paths 446-452may be detected by the sense amplifier 454 and the cross coupled devicemay cause the voltages to resolve towards power. In response toidentifying that the voltages are being resolved towards power, thesense amplifier 454 is able to provide the value of one back to thevolatile storage element 402.

In the present example, the bridge 406 includes sixteen MTJs 412-442.However, in some implementations, the number of resistive elements orMTJs associated with the bridge 406 are related to the number of MTJsrequired to distribute the bias voltages between the MTJs withoutcausing one or more of the values stored on the MTJs to be flipped orswitched. Therefore, it should be understood that in otherimplementations the bridge 406 may have additional MTJs may be utilizedto store the state of the volatile storage element 402 when the volatilestorage element 402 is powered down.

FIG. 5 illustrates a physical diagram showing select components of thebridge 406 of FIG. 2 according to some implementations. In theillustrated example, the MTJs 412-442 are shown along a metal layer oras part of a physical layout including the interconnections via thebridge top 408, the bridge bottom 410, the drive paths 456-470, the outpath P1 446, the out path N1 448, the out path P2 450, the out path N2452, the midpoint path P 444, and the midpoint path 442.

For instance, the MTJ 426 and the MTJ 424 are connected via the drivepath 468, the MTJ 424 and the MTJ 422 are connected via the out path P2450, the MTJ 422 and the MTJ 420 are connected via the drive path 464,the MTJ 420 and the MTJ 418 are connected via the midpoint path 440, theMTJ 418 and the MTJ 416 are connected via the drive path 460, the MTJ416 and the MTJ 414 are connected via the out path P1 446, the MTJ 414and the MTJ 412 are connected via drive path 456, the MTJ 412 and theMTJ 428 are connected via the bridge top 408, the MTJ 428 and the MTJ430 are connected via drive path 458, the MTJ 430 and the MTJ 432 areconnected via the out path N1 448, the MTJ 432 and the MTJ 434 areconnected via drive path 462, the MTJ 434 and the MTJ 436 are connectedvia the midpoint path 442, the MTJ 436 and the MTJ 438 are connected viathe drive path 466, the MTJ 438 and the MTJ 440 are connected via theout path N2 452, the MTJ 440 and the MTJ 442 are connected via the drivepath 470, and the MTJ 442 and the MTJ 426 are connected via bridgebottom 410.

In general, when a value of zero is to be stored by the bridge 406, awrite driver, such as the read bias/write driver 472 of FIG. 4, causesthe drive paths 456, 462, 464, 470 to be driven high and the drive paths458, 460, 466, and 468 to be driven low resulting in the MTJs 412, 414,420, 422, 432, 434, 440, and 442 being placed in a high state and theMTJs 416, 418, 426, 424, 428, 430, 436, 438 being placed in a low state.In other words, when the value of zero is to be stored by the bridge406, the drive paths 456, 462, 464, 470 are driven high, and the drivepaths 458, 460, 466, and 468 are driven low thereby applying a downcurrent along the MTJs 416, 418, 426, 424, 428, 430, 436, 438 and an upcurrent along the MTJs 412, 414, 420, 422, 432, 434, 440, and 442.

Likewise, when a value of one is to be stored by the bridge 106 thewrite driver causes the drive paths 456, 462, 464, 470 to be driven lowand the drive paths 458, 460, 466, and 468 to be driven high resultingin the MTJs 412, 414, 420, 422, 432, 434, 440, and 442 being placed in alow state and the MTJs 416, 418, 426, 424, 428, 430, 436, 438 beingplaced in a high state. In other words, when the value of one is to bestored by the bridge 406, the drive paths 456, 462, 464, 470 are drivenlow and the drive paths 458, 460, 466, and 468 are driven high therebyapplying an up current along the MTJs 416, 418, 426, 424, 428, 430, 436,438 and a down current along the MTJs 412, 414, 420, 422, 432, 434, 440,and 442.

In some implementations, by configuring the resistive state of the MTJs412-442 by applying voltages to the drive paths 152-158 in the mannerdescribed above, the bridge 406 may be configured to store a value(e.g., one or zero) detectable by a sense amplifier, such as the senseamplifier describe below with respect to FIG. 6, when the bridge top 408and the bridge bottom 410 are biased by a read bias component, such asread bias/write driver 472 of FIG. 4.

For instance, in the illustrated example, the read bias component maydrive the bridge top 408 high and the bridge bottom 410 low associatinga first voltage with the out path P1 446, a second voltage with the outpath N1 448, a third voltage with the out path P2 450, and a fourthvoltage with the out path N2 452 based on the resistive states of theMTJs 412-442, as configured in response to the write driver applyingvarious voltages to the drive paths 456-470. In this instance, thefirst, second, third, and fourth voltages may be detected by one or moresense amplifiers. The sense amplifier may determine the value stored onthe bridge 406 based at least in part on the differential between thefirst voltage, the second voltage, the third voltage, and the fourthvoltage. For example, the sense amplifier may include one or more crosscoupled devices configured to cause the voltage detected to resolve toeither power or ground based on the voltages associated with each of theout paths 446-452, and, thereby the sense amplifier may output a digitalvalue, such as a zero or one corresponding to either power or ground.

FIG. 6 illustrates a partial logic diagram and a partial circuit diagramshowing select components of the sense amplifier 454 of FIG. 4 accordingto some implementations. In general, the sense amplifier 454, inresponse to receiving an enable signal (such as the recall signal 474 ofFIG. 4) on the enable path 602, is configured to determine or detect avoltage differential stored on the bridge associated with thenonvolatile storage element of FIGS. 4 and 5 when the bridge top andbridge bottom are biased by a read bias component. The sense amplifier454 identifies the value stored on the bridge based at least in part onthe voltages associated with the out paths 446-452. For instance, in theillustrated example, the sense amplifier 454 receives a first voltageassociated with out path P1 446 and a second voltage associated with theout path P2 450 at an impedance path 604 and a third voltage associatedwith out path N1 448 and a fourth voltage associated with the out pathN2 452 at an impedance path 606. The sense amplifier 454 is able toresolve the voltages to either power or ground and, thereby, determine adigital value (e.g., zero or one) associated with nonvolatile storageelement 402.

In the illustrated example, the sense amplifier 454 includes two crosscoupled devices 608 and 610 coupled to the impedance paths 604 and 606.The cross coupled devices 608 and 610 are arranged such that based onthe voltages associated with the paths 446-452, the combined voltageresolves to either ground or power (e.g., zero or one). In this manner,the sense amplifier 454 may output a value along data out path 612 tothe volatile storage element 402, as a digital signal of either zero orone upon power up of the electronic device.

In one example, assume that the voltage associated with the out path P1446 and the out path P2 450 are higher than the voltages associated withthe out path N1 448 and the out path N2 452, respectively. In thisexample, the voltage received at a transistors controlling the impedancepath 604 is larger than the voltage received at a transistorscontrolling the impedance path 606 causing a voltage associated with theimpedance path 604 to be pulled down further than a voltage at theimpedance path 606. When this happens, a feedback loop associated withthe cross coupled devices 608 and 610 cause the voltage associated withthe impedance path 604 to tend towards ground and the voltage associatedwith the impedance path 606 to tend towards power, which the senseamplifier 454 detects as a high signal or a one that may be output tothe volatile storage element 402.

In an alternative example, assume that the voltage associated with theout path P1 446 and the out path P2 450 are lower than the voltagesassociated with the out path N1 448 and the out path N2 452,respectively. In this example, the voltage received at the transistorscontrolling the impedance path 604 is smaller than the voltage receivedat the transistors controlling the impedance path 606 causing a voltageassociated with the impedance path 606 to be pulled down further than avoltage at the impedance path 604. When this happens, a feedback loopassociated with the cross coupled devices 608 and 610 cause the voltageassociated with the impedance path 604 to tend towards power and thevoltage associated with the impedance path 606 to tend towards ground,which the sense amplifier 454 detects as a low signal or a zero that maybe output to the volatile storage element 402. Thus, in this manner thesense amplifier 454 is capable of translating the high and low signalsstored on the MTJs of the bridge associated with the nonvolatile storageelement into a digital signal that may be provided to the volatilestorage element 402 when power is restored to the circuit.

FIG. 7 illustrates a partial logic diagram and a partial circuit diagramshowing select components of circuit 700 for use a physical unclonablefunction (PUF) according to some implementations. In some cases, due tothe fact that the stored magnetic orientations associated with thenonvolatile flip flop, logic gate, and/or other circuitry is difficultto detect, the nonvolatile flip flop, logic gate, and/or other circuitrymay be utilized as one or more security features for a device, such as adevice specific fingerprint. For instance, in some implementations, thenonvolatile flip flop, logic gate, and/or other circuitry describedabove with respect to FIGS. 1-6, may be used to create a PUF that may beused as a signature or fingerprint unique to each device incorporatingthe flip flop, logic gate, and/or other circuitry.

For instance, in the illustrated example, the circuit 700 includes anonvolatile storage element 702 including a bridge 704 connected to asense amplifier 706. In general, the bridge 704 has a bridge top 708 anda bridge bottom 710. The bridge 704 may be formed from multipleresistive elements, such as tunnel junctions or MTJs 712-742, arrangedin two columns, as described above with respect to FIG. 4. The bridgeincludes a midpoint path P 744 associated with the first column and amidpoint path N 746 associated with the second column. The midpoint pathP 744 is arranged such that four MTJs 712-718 are positioned above themidpoint path P 744 and four MTJs 720-726 are positioned below themidpoint path P 744. Likewise, the midpoint path N 746 is arranged suchthat four MTJs 728-734 are positioned above the midpoint path N 744 andfour MTJs 736-742 are positioned below the midpoint path N 746.

The sense amplifier 706 is connected to both columns of the bridge 704.For instance, the sense amplifier 706 is connected to the first columnvia the midpoint path P 744 and the second column via the midpoint pathN 746. In this manner, the sense amplifier 706 is able to detect avoltage differential between a voltage associated with the midpoint pathP 744 and the midpoint path N 746, for instance as described above withrespect to FIG. 3. In some cases, the transistors associated with thesense amplifier 706 may be sized to compensate variation in operation ofthe transistors of the sense amplifier 706 and the MTJs 712-742 astemperature and voltages fluctuate. For instance, in one particularexample, the transistors of the sense amplifier 706 may be larger thanthe MTJs 712-742.

In general, the MTJs 712-742 may be set or configured by applying drivevoltages over the bridge as discussed above with respect to FIG. 4. Asdiscussed above, when the drive voltages are applied by a write driverthe MTJs are configured such that in each column, half the MTJs are highand half of the MTJs are low. For example, in a first configurations,the MTJs 712, 714, 720, 722 in the first column may be high, while theMTJs 716, 718, 724, and 726 may be low. Likewise, in the firstconfiguration, the MTJs 732, 734, 740, and 742 in the second column maybe high, while the MTJs 728, 730, 736, and 738 may be low.Alternatively, in a second configuration, the MTJs 712, 714, 720, 722 inthe first column may be low, while the MTJs 716, 718, 724, and 726 maybe high. Likewise, in the second configuration, the MTJs 732, 734, 740,and 742 in the second column may be low, while the MTJs 728, 730, 736,and 738 may be high. In either configuration, if resistive state of eachof the MTJs in the high state have the same resistance and each of theMTJs in the low state have the same resistance, there would be novoltage differential between the voltage associated with the midpointpath P 744 and the midpoint path N 746 and, thereby, no differentialdetected by the sense amplifier 706.

However, due to the natural variation in the MTJ tunnel barrierthickness and MR and, thereby, the resistance of each states of the MTJs712-746, a voltage differential will occur between the voltageassociated with the midpoint path P 744 and the voltages associated withthe midpoint path N 746. The sense amplifier 706 may then resolve thedifferential into either a high or low state as described above withrespect to FIG. 3, to generate a unique output value (e.g., either zeroor one). In this manner, a random value may be generated using thebridge 704, that unless known may be difficult to detect.

In some particular examples, the voltage associated with the midpointpath P 744 and the voltage associated with the midpoint path N 746 maybe so similar or equitant that sense amplifier 706 is unable to reliablydetect or identify the voltage differential and, thereby is unable toreliably resolve the voltage differential associated with the bridge704. Since the bridge 704 has two states or arrangements, when thevoltage differential is unreliable in the first state, the write drivemay switch the state, as in most cases the sense amplifier 706 is ableto reliably detect a voltage differential between the voltagesassociated with the midpoint path P 744 and the midpoint path N 746 inat least one of the two states.

In some cases, by incorporating multiple circuits 700 into in a device,a PUF signature may be generated based on the natural output of high andlow signals associated with each of the circuits 700. For example, arandom series of bits having value of either zero or one. Further, sincethe PUF signature is based on a natural variation in the resistance ofeach state of the MTJs 712-742, the PUF signature is not easilyidentified or duplicated.

FIG. 8 illustrates a block diagram of a device 800 utilizing a PUF aspart of an unlock key according to some implementations. In general, thedevice 800 includes multiple PUF circuits 802-808 configured to producea random value (e.g., either zero or one), as describe above withrespect to FIG. 7. For example, the PUF circuit 802 may generate a firstdigit 810 of an unlock key for unlocking the device 800, the PUF circuit804 may generate the second digit 812 of the unlock key, the third PUFcircuit 806 may generate the third digit 814 of the unlock key, and soforth until the nth PUF circuit 808 generates the nth digit 816 of theunlock key. By generating the unlock key in this manner, the device 800now has an identifier that was neither programmed nor alterable and,therefore, difficult to hack.

The device 800 may also include an unlock circuit 818 configured toreceive an unlock code 820 as an input either from a user of the device800 or an external source associated with the device 800. In some cases,the unlock circuit 818 is configured to compare the unlock code 820 tothe unlock key generated based on the digits 810-816. In other cases,the unlock circuit 818 may unlock when a predetermined percentage of thedigits of the unlock code match the digits 810-816, for instance incases that the resistive state of the MTJs in the PUF circuits 802-808vary sufficiently as the temperature fluctuates and causes erroneousreads on some of the MTJs.

Since each of the PUF circuits 802-808 include a bridge of MTJs thathave two states (e.g., high and low), in some particular examples, theunlock key formed from the digits 810-816 may be variable. For instance,a write driver may flip the state of the bridge associated with selectedPUF circuits 802-808 after each successful code 820 entry, such that theunlock key formed from the digits 810-816 changes in a pattern known toowner of the device after each use. In other examples, the write drivermay flip the state of the bridge associated with the selected PUFcircuits 802-808 based on a length of time, a number of correct keyentries, the time of day, among others. In these examples, the user maybe supplied with a key timer that provides them with the correct unlockcode 820 during the associated period of time.

In some instance, such as the illustrated example, each of the PUFscircuits 802-808 may be associated with an ECC circuit 822 to improvethe overall reliability of the PUFs circuits 802-808. For example, insome cases the state of one or more of the MTJs associated with the PUFscircuit 802-808 may be disturbed. When this happens, the digits 810-816output by one or more of the PUFs circuits 802-808 may be erroneous,however, by utilizing the ECC circuit some of the erroneous reads may becorrected before the unlock key is compared with the unlock code 820,thereby improving the reliability of the PUFs circuits 802-808. The ECCcircuit 822 may receive parity bit information stored in additionalnonvolatile elements (not shown), such as nonvolatile storage element104 of FIG. 1 or nonvolatile storage element 404 of FIG. 4. Alternately,the parity bit information may be stored as the state associated withone or more of the PUF circuits 802-808.

FIGS. 9 and 10 are flow diagrams illustrating example processes ofnonvolatile logic gates used for storing a state and/or as a PUFcircuit. The processes are illustrated as a collection of blocks in alogical flow diagram, which represent a sequence of operations, some orall of which can be implemented in hardware, software or a combinationthereof. In the context of software, the blocks representcomputer-executable instructions stored on one or more computer-readablemedia that, which when executed by one or more processors, perform therecited operations. Generally, computer-executable instructions includeroutines, programs, objects, components, data structures and the likethat perform particular functions or implement particular abstract datatypes.

The order in which the operations are described should not be construedas a limitation. Any number of the described blocks can be combined inany order and/or in parallel to implement the process, or alternativeprocesses, and not all of the blocks need be executed. For discussionpurposes, the processes herein are described with reference to theframeworks, architectures and environments described in the examplesherein, although the processes may be implemented in a wide variety ofother frameworks, architectures or environments.

FIG. 9 illustrates an example flow diagram showing an illustrativeprocess 900 for maintaining a state associated with a nonvolatilestorage element, such as the nonvolatile storage elements 104 and 404 ofFIGS. 1 and 4, through a powered down period according to someimplementations. For example, MRAM or ST-MRAM elements may be utilizedto generate nonvolatile flip flops, nonvolatile logic gates, and/orother nonvolatile circuits for maintaining a state while a device orportion of a device is powered off, as described above with respect toFIGS. 1-6.

At 902, a write driver associated with the nonvolatile storage elementreceives a store signal and a value (e.g., zero or one) to maintainthrough a powered down period. For instance, the write driver mayreceive the store signal and the value from a voltage storage elementand/or from another element associated with the circuit incorporatingthe nonvolatile storage element and the volatile storage element.

At 904, the write driver drives a first plurality of drive paths highand a second plurality of drive paths low to arrange a bridge of MTJsassociated with the nonvolatile storage element to store a stateassociated with the value. In some cases, the write driver selects thedrive paths to drive high and the drive paths to drive low based atleast in part on the value to be stored. For example, as describedabove, if the bridge includes two columns of four MTJs and the value tobe stored is a zero, the write drive may drive the paths high and low inorder to arrange the first column of MTJs from the bridge top to thebridge bottom as high, high, low, low, high, high, low, low and thesecond column of MTJs from the bridge top to the bridge bottom as low,low, high, high, low, low, high, high. Alternatively if the value to bestored is a one, the write drive may drive the paths high and low inorder to arrange the first column of MTJs from the bridge top to thebridge bottom as low, low, high, high, low, low, high, high and thesecond column of MTJs from the bridge top to the bridge bottom as high,high, low, low, high, high, low, low.

At 906, a sense amplifier and a read bias component receive a recallsignal. For instance, the recall signal may be triggered in response todetecting that the volatile storage element has power once more. Inother instances, the recall signal may be received from various otherelements of the circuit incorporating the nonvolatile storage elementand the volatile storage element.

At 908, a read bias component may bias the top of the bridge and thebottom of the bridge. For example, the bias circuit may drive the bridgetop high and the bridge bottom low in order to create one or morevoltage differentials along the bridge.

At 910, the sense amplifier detects one or more voltage differentialsbetween at least one out path associated with the first column of thebridge and at least one out path associated with the second column ofthe bridge. For example, the sense amplifier may have one or more crosscoupled devices that cause the voltages associated with out paths toresolve to either power or ground (e.g., one or zero).

At 912, the sense amplifier outputs the value to the volatile storageelement. For example, if the voltages had resolved to power, the senseamplifier may output a digital value, such as a one. Likewise, if thevoltages had resolved to ground, the sense amplifier may output adigital value, such as zero. Thus, by utilizing a nonvolatile storageelement including a bridge formed from MTJs which may be configured bythe write driver, a state or value may be stored through a low power orpowered off period.

FIG. 10 illustrates an example flow diagram showing an illustrativeprocess 1000 for utilizing a nonvolatile storage element as a PUFaccording to some implementations. For instance, due to the fact thatthe stored magnetic orientations associated with the nonvolatile storageelements described herein are difficult to detect, the nonvolatilestorage elements may be utilized as one or more security features, insome implementations. For example, the nonvolatile storage elements maybe used to create PUF circuits that may be used in combination togenerate a device signature, fingerprint, or unlock code that is hard todetect, difficult to alterable, and unique to each device. In somespecific implementations, an error correction code (ECC) circuit may beused in conjunction with the PUF circuit to improve the consistency ofthe output response and, thereby, the overall reliability of the PUFcircuit.

At 1002, an unlock circuit, such as the unlock circuit 818 of FIG. 8,receives a unlock code from another circuit or a user associated withthe device. For instance, a user of a device may enter an unlock codebefore the device allows the user to access at least a portion of thefeatures associated with the device.

At 1004, the unlock circuit receives individual digits of an unlock keyfrom a plurality of PUF circuits. For example, each of the PUF circuits,described above with respect to FIG. 8, are capable of generating a highor low voltage that is detectable and interpreted by a sense amplifieras a zero or one. Since the voltage either high or low is dependent onthe natural variations within the MTJs that form the PUF circuits, thevalue either zero or one is random, hard to alter, and difficult todetect. In this example, the unlock circuit receives different digits ofthe unlock key from each of the PUFs circuits and, thereby a random keythat is unique to the device.

At 1006, the unlock circuit compares the unlock code to the unlock keyand, at 1008, the unlock circuit allows access to the device when morethan a threshold number of digits of the unlock code and the unlock keymatch. In some cases, the threshold may be one hundred percent of thedigits match or an exact match between the unlock key and the unlockcode. In other examples, a smaller percent such as ninety five percent,may need to match before the unlock circuit allows access to the device,as in some cases variations in temperature and/or voltage may cause oneor more of the PUF circuits to generate an unexpected value. In someparticular instances, the unlock circuit may receive the unlock codefrom the external source and multiple versions of the unlock key fromthe PUF circuits and unlock if the unlock code matches above a secondthreshold number of the unlock keys.

Although the subject matter has been described in language specific tostructural features, it is to be understood that the subject matterdefined in the appended claims is not necessarily limited to thespecific features described. Rather, the specific features are disclosedas illustrative forms of implementing the claims.

What is claimed is:
 1. A circuit comprising: a volatile storage elementconfigured to store a state when the circuit is in a first mode; anonvolatile storage element having a first column of resistive elementshaving a first midpoint, wherein the first midpoint is between first andsecond pluralities of resistive elements of the first column, and asecond column of resistive elements having a second midpoint, whereinthe second midpoint is between third and fourth pluralities of resistiveelements of the second column; a write driver configured to detect thestate and to configure a current resistive state associated with thenonvolatile storage element by applying a plurality of drive voltages tothe nonvolatile storage element in response to receiving a store signal,the store signal indicating the circuit is transitioning from the firstmode to a second mode; and a sense amplifier configured to (i) determinethe current resistive state when the circuit is in the second mode basedat least in part on a voltage differential between the first midpointand the second midpoint of the nonvolatile storage element and (ii)output the current resistive state to the volatile storage element inresponse to receiving a recall signal, the recall signal indicating thecircuit is transitioning from the second mode to the first mode.
 2. Thecircuit as recited in claim 1, wherein the first mode is a powered modeand the second mode is at least one of a sleep mode, low power mode, oroff mode.
 3. The circuit as recited in claim 1, further comprising aread bias component to bias the nonvolatile storage element in responseto receiving the recall signal, the biasing causing the voltagedifferential on the nonvolatile storage element to be detectable by thesense amplifier.
 4. The circuit as recited in claim 3, wherein the readbias component and the write driver are a same component of the circuit.5. The circuit as recited in claim 3, wherein the nonvolatile storageelement includes: a bridge top coupled to a first resistive element ofthe first column and a first resistive element of the second column, thebridge top configured to receive a first bias voltage from the read biascomponent; a bridge bottom coupled to a second resistive element of thefirst column and a second resistive element of the second column, thebridge bottom configured to receive a second bias voltage from the readbias component; a first out path coupled to the first column; a secondout path coupled to the second column; a first drive path and a seconddrive path associated with the first column of resistive elements, thefirst drive path configured to receive a first drive voltage from thewrite driver and the second drive path configured to receive a seconddrive voltage from the write driver; and a third drive path and a fourthdrive path associated with the second column of resistive elements, thethird drive path configured to receive a third drive voltage from awrite driver and the fourth drive path configured to receive a fourthdrive voltage from the write driver, wherein the first drive voltage,the second drive voltage, the third drive voltage, and the fourth drivevoltage configure a state of the resistive elements of the first columnof resistive elements and the resistive elements of the second column ofresistive elements to associate a first output voltage with the firstout path and a second output voltage with the second out path when thebridge top and the bridge bottom are biased and the sense amplifierdetermines the state based at least in part on the first output voltageand the second output voltage.
 6. The circuit as recited in claim 5,wherein the nonvolatile memory includes a first resistive mode and asecond resistive mode, and wherein the first resistive mode and thesecond resistive mode are configurable by the write driver.
 7. Thestorage element as recited in claim 5, wherein: the first out path iscoupled to the first column at the first midpoint; and the second outpath is coupled to the second column at the second midpoint.
 8. Thecircuit as recited in claim 5, wherein the resistive elements aremagnetic tunnel junctions.
 9. A circuit comprising: a volatile storageelement having a state; a nonvolatile storage element having a firstcolumn of resistive elements having a first midpoint, wherein the firstmidpoint is between first and second pluralities of resistive elementsof the first column, and a second column of resistive elements having asecond midpoint, wherein the second midpoint is between third and fourthpluralities of resistive elements of the second column; a write driverconfigured to detect the state of the volatile storage element and toplace the nonvolatile storage element in a first resistive mode byapplying a plurality of drive voltages to the nonvolatile storageelement in response to receiving a store signal; and a sense amplifierconfigured to determine a voltage differential between the firstmidpoint and the second midpoint in response to receiving a recallsignal, the voltage differential representative of a current resistivestate associated with the nonvolatile storage element.
 10. The circuitas recited in claim 9, wherein the voltage differential represents oneof a first resistive mode or a second resistive mode associated with thenonvolatile storage element.
 11. The circuit as recited in claim 9,further comprising a read bias component to bias the nonvolatile storageelement in response to receiving the recall signal, the biasing causingthe voltage differential on the nonvolatile storage element to bedetectable by the sense amplifier.
 12. The circuit as recited in claim11, wherein the read bias component and the write driver are a samecomponent of the circuit.
 13. The circuit as recited in claim 9, whereinthe nonvolatile storage element includes: a bridge top coupled to afirst resistive element of the first column and a first resistiveelement of the second column, the bridge top configured to receive afirst bias voltage; a bridge bottom coupled to a second resistiveelement of the first column and a second resistive element of the secondcolumn, the bridge bottom configured to receive a second bias voltage; afirst out path coupled to the first column at the first midpoint; asecond out path coupled to the second column at the second midpoint; andwherein the second column having a same number of resistive elements asthe first column of resistive elements and the sense amplifier iscoupled to the first out path and the second out path.
 14. The circuitas recited in claim 9, wherein the nonvolatile storage element includes:a first drive path and a second drive path associated with the firstcolumn of resistive elements, the first drive path configured to receivea first drive voltage and the second drive path configured to receive asecond drive voltage; and a third drive path and a fourth drive pathassociated with the second column of resistive elements, the third drivepath configured to receive a third drive voltage and the fourth drivepath configured to receive a fourth drive voltage, wherein the firstdrive voltage, the second drive voltage, the third drive voltage, andthe fourth drive voltage configure a state of the resistive elements ofthe first column of resistive elements and the resistive elements of thesecond column of resistive elements to generate the voltagedifferential.
 15. The storage element as recited in claim 14, whereinthe first drive voltage, the second drive voltage, the third drivevoltage, and the fourth drive voltage cause: a first half of theresistive elements of the first column to be placed in a low resistivestate and a second half of the resistive elements of the first column tobe placed in a high resistive state; and a first half of the resistiveelements of the second column to be placed in a high state and a secondhalf of the resistive elements of the second column to be placed in alow resistive state.